A significant amount of resources are allocated to telecommunications network architectures, as well as OAM&P to ensure highly reliable data communications. Reliable performance depends on reliable and timely performance monitoring for verification and protection when required. Optical fiber based telecommunication systems in particular require accurate performance monitoring solutions given the higher traffic capacities and related costs.
The Synchronous Optical Network (SONET) standard formulated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI) provides a common policy for optical transport. The standard incorporates information protocols that define how overhead services are used by transmission equipment to provide a range of performance monitoring services. Protection switching and performance monitoring services use statistical measures to automatically isolate and identify areas of the network experiencing signal degrade (SD). Fault management services not only detect and process faults locally but also signal measured phenomena quickly to neighboring equipment that may be susceptible to faults detected elsewhere in the network.
Briefly, digital communication over a SONET network is based on a synchronous transport system frame protocol, STS-n. The SONET frame is partitioned into a transport overhead (TOH) part consisting of three columns and nine rows, and a synchronous payload envelope (SPE) part consisting of 87 columns, one column of which is for path overhead (POH). TOH consists of three columns and three rows of section overhead (SOH) and three columns and six rows of line overhead (LOH). Of the 810 bytes in the 9 row by 90 column byte stream in each STS-1 frame, 36 bytes are thus reserved for SONET overhead information. The data therein provides the control information for each of three specific layers (signal, line and path) within the SONET standard. Each SONET frame is transmitted at a rate of 8 kHz (i.e. every 125 μsec). Transmissions rates are increased by increasing the number of STS-1 frames transmitted at the 8 kHz rate. Thus for the optical carrier (OC) rate OC-48, 48 STS-1 frames, (STS-1#1, STS-1#2 . . . STS-1#48, collectively referred to as a STS-48 frame), are transmitted every 125 μsec.
One performance monitoring measure of a telecommunication network's quality of service is the bit error rate (BER) of the traffic being carried through the network. Each of the section, line and path layers represented in the SONET STS-1 overhead includes a one byte compounded bit interleaved parity (BIP-8) field (B1, B2 and B3 respectively) that can be used to calculate BER for transmitted signals at points throughout the network. As an example, and as is understood by those skilled in the art, line BER can be calculated at a receiving site by comparing the BIP-8 value received in a B2 byte to a BIP-8 value determined at the receiving site from the bytes of the STS-1 frame element associated with the B2 value.
Bellcore, an initiator of the SONET standard, has published Generic Requirement GR-253-CORE, incorporated by reference herein, setting out, among other things, error count statistics and maximum detection times that each SONET network element (NE) is expected to maintain, by layer. The statistics are collected for a current measured period and for historical periods. For example, included in the statistics collected is a measure of Coding Violations (CVs) representing a raw count of bit errors detected using the BIP-8 for each layer. These statistics are further collected in time-interval buckets for historical performance monitoring. The buckets accumulate 15 minute and 24 hour interval data. Each NE maintains up to eight hours of 15-minute interval data and 48 hours of 24-hour data. The data may be collected by a central data management system for longer-term use. From these accumulated statistics, operators and network users can review both the recent and historic behaviors of their network. Moreover, protection switching activities may be performed to identify and avoid signal degrade problems.
Trite as it may sound, it is very important to maintain timely and accurate BER calculation abilities when transporting SONET frames.
Applicant's U.S. Pat. No. 5,841,760, which issued Nov. 24, 1998 to Martin et al., discloses a transport node suitable for SONET telecommunications. The node comprises a pair of transparent multiplexer/demultiplexers provided at two sites and connected by a high-rate span. The transport node provides continuity of all tributary (trib) signals and maintains one or more lower bit rate linear or ring trib systems through the higher bit rate span. Each lower bit rate trib system operating through the transport node operates as if it were directly connected without the higher bit rate midsection.
For the forward direction of the traffic, the T-Mux comprises a multi-channel receiver for receiving the trib signals (e.g. four OC-48 signals), processing each trib signal to derive a respective trib SPE and a trib OH. The trib SPEs are multiplexed into a supercarrier SPE and the trib OHs signals are processed to generate a supercarrier OH. A supercarrier transmitter maps the supercarrier SPE and the supercarrier OH into a supercarrier signal (e.g. an OC-192 signal) and transmits the signal over the high-rate span. Reverse operations are effected for the reverse direction of traffic. With this transport node, an entire ring trib system, for example, does not have to be upgraded to a higher line rate due to fiber exhaust on a single span.
In order to maintain transparency, and avoid any provisioning or additional costs at trib sites served by the transport node, a trib site receiving trib signals through the transport node should be able to compile performance monitoring statistics, such as BER, for the trib signal it receives as if it received the signal directly. That is, as if the receiving trib site was located at the upstream end of the transport node and not the downstream end. Moreover, the receiving trib site must be able to calculate such statistics in a timely manner. Such functionality is also necessary because, to maintain transparency, the transport node does not perform protection switching.
Under the SONET standard, a particular STS-1 received by a trib site from a T-Mux at the downstream end of the transport node is not necessarily identical to the corresponding STS-1 received by the upstream T-Mux. There are at least three reasons that may explain the differences. First, there may be errors introduced over the high-rate span or, second, over the low rate span between the downstream T-Mux and the trib site. Third, overhead bytes, particularly LOH payload pointers H1, H2 or H3 used to specify the beginning of an SPE within the frame may be adjusted by either the upstream or downstream T-Mux. Such adjustments are necessary to accommodate frequency offsets between a received STS-1 frame and the local system frame. Any such adjustment causes a further change to B2 computed for the particular STS-1 frame for transmission to the trib receiving site. Thus, if the receiving site merely calculated performance statistics on the frames it received, the statistics would reflect the presence of the T-Mux to a substantial degree.
It is therefore necessary to passthrough upstream trib signal degrade information such as BER as determined at the upstream end of the transport node to the downstream end for propagation to the respective receiving trib sites.
Martin et al. describe a method for determining BER for each trib signal at the upstream T-Mux and passing the error rate through to the downstream T-Mux by encoding a message byte in unused STS-n frame overhead. The detected error rate is then propagated to the receiving trib sites by inserting an equivalent number of errors into outgoing signals. Errors may be introduced by inverting an equivalent number of bits of B2. The method provides a B2 error rate transfer with one-half decade accuracy.
The integration time allowed for detection of an incoming BER is of an order of minutes according to Bellcore GR-253-CORE. A delay in measurement and error transfer propagates to the downstream trib sites and could effectively double the BER detection time at those sites.
The delay in error propagation also affects the time at which the bit error information is reported by the downstream site's Performance Monitoring (PM) system. Single bit errors that may be reported by an OC-192 T-Mux may never be propagated to a downstream OC-48 site. It is therefore desirous to provide signal degrade propagation that is both timely and necessarily reflective of measured error rates.
As a result, there is a need to provide an improved method and system for propagating signal degrade information in transparent multiplexer/demultiplexer (T-Mux) systems.